The present invention relates to the field of liquid or semi-liquid waste disposal and more particularly pertains to a novel process for disposing of liquid or semi-liquid wastes containing soluble and insoluble toxic materials capable of being leached into the surrounding environment when the wastes are exposed to ambient moisture thereby resulting in environmental pollution of the surrounding environment.
For many years, the majority of liquid and semi-liquid wastes containing soluble and insoluble toxic materials, such as industrial chemical process wastes, particularly aqueous sludges from the manufacture of phosphoric acids containing soluble and insoluble arsenic compounds, sulfur compounds, etc., were disposed of by concentrating the wastes in settling pools to a practicable solids level and then merely dumping or burying the concentrated materials at suitable landfill locations. However, due to present Federal and State regulations, these techniques have generally been discontinued for the exposure of such disposed wastes causes many of the toxic materials to leach out into nearby streams and underground water systems thereby creating environmental pollution.
There are several processes known for solidifying many types of liquid and semi-liquid waste materials to enable them to be disposed of as solids or substantially solid materials in or under suitable landfill sites. Generally speaking, such procedures include mixing the liquid or semi-liquid waste materials with inorganic mineral binding agents which react with the liquid of the wastes and set up to form substantially insoluble solid materials. Such procedures have been known and employed for many years; for example, U.S. Pat. No. 151,905 discloses a process for the manufacture of fertilizer which includes admixing night-soil with hydraulic cement or calcined plaster and pulverizing the resulting set solid. A more recent patent, U.S. Pat. No. 3,383,228, discloses a process for disposing of garbage and sewage in the form of solid blocks which includes a destructive distillation of the wastes, mixing the residual mass with a suitable binding agent such as sodium silicate and compacting the resulting admixture into blocks. U.S. Pat. No. 2,809,118 teaches admixing sewage sludges with water glass (sodium silicate), a hydratizing chemical such as sodium carbonate, ammonium carbonate, sodium bicarbonate, ammonium phosphate, and the like, and a clay material to form substantially solid plastic materials suitable for compacting into constructional materials.
Further, U.S. Pat. No. 3,837,872 describes a process for treating liquid and semi-liquid wastes to render them non-polluting which includes admixing with the liquid wastes an alkali metal silicate such as sodium silicate and a silicate setting agent such as Portland cement, lime, gypsum, etc., and calcium chloride which results in the formation of solid materials having physical characteristics ranging from dirt-like to substantially rock-like solids.
However, known prior art processes for solidifying liquid and semi-liquid wastes employing conventional mineral binding agents mentioned hereinbefore in accordance with known conventional techniques have been found to leave much to be desired when employed in the disposal of many types of liquid and semi-liquid wastes containing soluble and insoluble toxic materials, such as aqueous sludges from the manufacture of phosphoric acids which contain soluble and insoluble arsenic compounds, sulfur compounds, etc. The employment of such conventional mineral binding agents, only a few of which have been mentioned hereinabove, in accordance with known conventional techniques, results in the formation of solids which are highly porous, many of which being highly particulated. Moreover, it has been found that processes employing alkali silicates, e.g. sodium silicate, as a binding agent alone or in combination with other binding agents result in solids having low compressive strength which are readily particulated thereby providing additional surface area exposure to the environment. The resulting solids render conventional waste solidifying procedures unsatisfactory in the disposal of many types of liquid and semi-liquid wastes containing soluble and/or insoluble toxic materials for these materials are readily leached out of the solidified wastes when they are exposed to moisture into the surrounding environment thereby causing environmental pollution. For example, the process described in U.S. Pat. No. 3,837,872 which describes the use of an alkali silicate and a silicate setting agent has been found to leave much to be desired in the disposal of semi-liquid sludges from the manufacture of phosphoric acid. The resulting solids produced by the process are easily broken down to a dirt-like consistency from which toxic arsenic materials can be readily leached out at environmentally unacceptable high levels.
We have now discovered a novel process for permanently disposing of liquid and semi-liquid wastes containing soluble toxic materials which effectively prevents the leaching of the toxic materials into the surrounding environment and thereby effectively eliminates pollution of surrounding streams, underground water systems and the like. The present invention is based upon the discovery that such wastes, particularly aqueous sludge wastes from the manufacture of phosphoric acids and similar industrial and chemical process wastes, can be solidified to a contiguous rock-like solid condition having high strength and low porosity which entraps both insoluble and soluble toxic materials therein thereby preventing leaching when exposed to ambient moisture by admixing with the wastes a critical amount of a hydraulic cement. Most unexpectedly, the amount of hydraulic cement employed has been found to be critical in preventing leaching of toxic materials from solidified wastes; these findings were surprising in view of the teachings of the prior art.